I've been using Stihl Ultra since it first came out when I got my first FS91 4-Mix. And that would had to have been sometime around 2003-04 I think. No problems with anything running it... 2 FS-91's, an old FS80, the wifey's little Stihl M55 tiller, and my 3 Husky saws. Never plugged a screen, fouled a plug, or anything like that. I do see just a trace of oil making it out of the mufflers I think. You know what that tells me? It's working when it goes in, and it's still working when it goes out.
And, I think it smells good.
The only thing I don't like about Ultra is how much the price has gone up.
If it ain't broke, don't fix it.